Keyboard sensor systems and methods

ABSTRACT

A sensing system for a keyboard. Each key sensor comprises passive and active resonant circuits. The passive resonant circuit has a resonant frequency and the active resonant circuit excites the passive resonant circuit at the resonant frequency. A sensor driver drives the active resonant circuit with an RF drive signal, a multiplexing system multiplexes the drive signal such that simultaneously driven key sensors are separated by at least (k−1) keys, and a detector detects a level of RF signal from a driven key sensor for sensing a position and/or velocity of a key.

FIELD OF THE INVENTION

The invention relates to sensing systems and methods for keyboards suchas musical instrument keyboards.

BACKGROUND

The applicant has previously described resonant circuit-based sensors inGB2494230A.

Musical keyboards for electronic instruments generally use a mechanicalswitch or similar contacting device to sense a single striking positionof a key wherein the closing of the switch is used to detect a note-onevent and the opening of the switch is used to detect a note-off event.More sophisticated versions of such instruments may use a plurality ofsuch switches.

Using mechanical sensors such as switches to sense the position of a keyon a musical keyboard has many disadvantages. Most musical keyboardsfeature a large number of keys, typically from 21 to 88 keys. To supportsuch a large number of keys the switches are commonly time-interleavedusing a multiplexing method. Such a multiplexing method combined withconnection jitter of the switches, also known as switch bounce, limitsthe speed at which the switches' connection and disconnection points canbe detected. In some cases the connection and disconnection of theswitches can be felt by the musician, which is highly undesirable. Inother cases the switches can become unreliable due to mechanical wearunless high-reliability switches are used, but such switches areexpensive. In all cases mechanical variations between the switchescauses variations in the response from one key to another key. Suchvariations are very difficult to remove by a calibration procedurebecause variation also occurs on an individual key when the key isactuated repeatedly. Furthermore, it is not possible to change the pointduring the movement of the key at which a switch connects anddisconnects without a mechanical change.

More advanced musical keyboards permit the pressure applied to the keyafter the note-on event has been issued to be used to control aspects ofthe musical sound. This pressure can be detected by a single pressuresensor element, for example a force-sensing resistor, where the pressurefrom all the keys on the keyboard is combined by mechanical coupling toexert pressure on the pressure sensor. Such a system is commonlyreferred to as monophonic aftertouch. A more desirable system permitsthe pressure applied to each individual key to be detectedindependently; this is referred to as pressure, or polyphonicaftertouch. Such a polyphonic aftertouch system is expensive because aseparate pressure sensor is used for each individual key.

Alternative detection methods for musical keyboards are known whichovercome many of the limitations of mechanical switches and pressuresensors, however these all still have undesirable properties.

Optical position sensing of a piano keyboard is described in US2009/0282962. However the performance of such systems is vulnerable todegradation by contamination and they need to be cleaned, orrecalibrated to retain optimal performance. Moreover, they can containdelicate optical elements such as shades or films with graduatedtransparency or reflectivity, which make them sensitive to shock andvibration with a corresponding reduction in long-term reliability.Furthermore, US2009/0282962 has an optical sensor and op-amp for eachkey which makes such an implementation expensive.

Magnetic sensors such as Hall probes where a permanent magnet is movedwith respect to the Hall probe are another way to detect the position ofa key on a musical keyboard. However such magnetic sensors are sensitiveto interference from external magnetic fields, to interference frommovement of nearby ferrous metals, and to changes in temperature, andthe sensors experience hysteresis which limits the accuracy andrepeatability of position sensing. Furthermore, the requirement of apermanent magnet and a magnetic sensor on each key makes such a solutiontoo expensive for most applications.

Capacitive position sensors are too sensitive to electro-magneticinterference and to the position of the musician's hands, and totemperature, which make their application for a musical keyboardimpractical.

U.S. Pat. No. 4,838,139 describes a musical keyboard with inductive coilsensors. In this arrangement each key carries a metal spoiler whichmoves towards/away from its associated sensor inductance coil. Howeverthe system is slow, too slow than desirable for an 88-note keyboard, andis affected by metal jewellery, and casework of the product andsupporting structures.

SUMMARY OF THE INVENTION

There is described a set of sensors for a keyboard, in particular thekeyboard of a keyboard instrument, in particular a piano-style keyboard.The keyboard has a plurality of keys. The set of sensors may be part ofa sensing system. Each sensor may comprise a passive resonant circuitfor mounting on a moving part of a key and an active resonant circuitfor mounting in a fixed, reference position, for example on part of thekeyboard or instrument. In implementations the passive resonant circuithas a resonant frequency and the active resonant circuit excites thepassive resonant circuit at the resonant frequency. Each sensor mayfurther comprise a detector, which may be shared between multiplesensors, to detect variation of a resonant signal in the active resonantcircuit with relative position of the active and passive resonantcircuits to thereby detect a position and/or velocity of the key. Thevariation may, in some implementations, be a variation in amplitude ofsignal in the resonant signal. The set of sensors may comprise sensorshaving two or more different resonant frequencies arranged such thatsensors having the same resonant frequency are non-adjacent when mountedto sense keys of the keyboard. Additionally or alternatively the one ormore coils of at least the active resonant circuits, and optionally alsoof the passive resonant circuits, may have windings in opposite sensesto reduce interference between sensors.

In some implementations, the system may be configured to providepolyphonic aftertouch by distinguishing between at least three differentkey positions, a first, note-off position, a second, note-on position,and a third, aftertouch position, wherein the aftertouch position isbeyond the note-on position and corresponds to additional pressureapplied to the key after depression and movement of a key beyond anend-stop position.

In implementations, one or more of the active resonant circuits maycomprise more than one coil and optionally the coils of the activeresonant circuit may be positioned next to one another along thelongitudinal direction defined by the key, such that the positionedcoils define a major length along the longitudinal direction and definea minor length perpendicular to the longitudinal direction. The one ormore coils of the active resonant circuit may further comprise twoprimary coils with opposing winding directions.

In implementations that passive resonant circuit may comprise one ormore coils, optionally with the one or more coils having opposingwinding directions. Optionally the one or more coils of the passiveresonant circuit may comprise two secondary coils wired in series withopposing winding directions. The secondary coils of the passive resonantcircuit may be primarily inductively coupled to different coils of theactive tuned resonant circuit respectively.

In implementations the active resonant circuits of adjacent keys may beoffset relative to an axis defined by the arrangement of the pluralityof keys. For example, an active resonant circuit of a first key of theplurality of keys and an active resonant circuit of a second key of theplurality of keys to be on either side of said axis are offset on eitherside of said axis.

There is also described a method of sensing the positions of a pluralityof keys, for example of a keyboard instrument. The method may compriseproviding each key with a sensor comprising a passive resonant circuitfor mounting, for example, on a moving part of a key and an activeresonant circuit for mounting, for example, in a fixed, referenceposition, for example part of the keyboard or instrument. In someimplementations the passive resonant circuit has a resonant frequency,the active resonant circuit exciting the passive resonant circuit at theresonant frequency. Each sensor may further have a detector, which maybe shared, to detect variation of a resonant signal in the activeresonant circuit with relative position of the active and passiveresonant circuits to detect a position and/or velocity of the key. Themethod may further comprise arranging the sensors to operate at two ormore different resonant frequencies arranged such that keyboard sensorshaving the same resonant frequency are non-adjacent. Additionally oralternatively and/or the method may further comprise reducinginterference between sensors by configuring one or more coils of atleast the active resonant circuits, and optionally also of the passiveresonant circuits, to have windings in opposite senses.

In one aspect there is provided a sensing system for a keyboard, forexample a musical instrument keyboard such as a piano-style keyboard.The sensing system may comprise a plurality of key sensors. Each keysensor may comprise a passive resonant circuit, for example for mountingon a moving part of a key, and an active resonant circuit, for examplefor mounting in a reference position. In implementations the passiveresonant circuit has a resonant frequency and the active resonantcircuit is configured to excite the passive resonant circuit at theresonant frequency. The sensing system may further comprise at least onesensor driver to drive the active resonant circuit with an RF drivesignal; this may be shared between multiple sensors. In implementationsthe sensing system may further comprise a multiplexing system, such asone or more multiplexers and/or demultiplexers, to multiplex the drivesignal such that simultaneously driven key sensors are (physically)separated by at least (k−1) keys, where (k−1) is an integer equal to orgreater than 1. Thus in implementations one key is not driven at thesame time as an adjacent key (or at the same time as a key at least kkeys away). The sensing system may further comprise at least onedetector, for example readout-circuitry and/or a microprocessor, todetect a level of RF signal from a driven key sensor. This may be usedfor sensing a position and/or velocity of a key associated with the keysensor. The at least one detector may detect variation of a resonant RFsignal in the active resonant circuit with relative position of theactive and passive resonant circuits; it may peak-detect the level of RFsignal.

At least the active resonant circuit, and optionally also the passiveresonant circuit, may comprise one, two or more coils, in particularwith windings in opposite senses. Thus, for example, the windings maygenerate magnetic fields in opposite senses, in particular balanced ormatched to cancel one another, in particular at long distances from thesensor

In implementations the combination of coils with opposite sense windings(and hence opposite sense currents/magnetic fields) and multiplexedsensor addressing facilitates the use of multiple sensors in closeproximity. Thus in implementations the windings in opposite senses areconfigured to generate balanced magnetic fields in opposite senses,which may cancel one another substantially completely at large distancesfrom a sensor, for example at a distance of at least ten times a maximumcoil dimension (which is not to say that the RF field from a sensor isundetectable at such a distance).

In some implementations the active resonant circuit comprises a pair of,or three or more, laterally adjacent pancake coils. (As used hereinreferences to two or more coils may be taken to include one coil withtwo or more windings, for example where the windings are in oppositesenses). The coils may be positioned next to one another longitudinallyalong a longitudinal direction defined by a key. The pancake coils maybe formed on a printed circuit board (PCB), which may be a flexible PCB,for ease of fabrication. The coils may, but need not have windings inopposite senses—some reduction in mutual interference may be obtainedsimply by employing this configuration of coils.

In implementations system, in particular the multiplexing system, isconfigured to damp the active resonant circuits of key sensors which arenot driven, for example by shorting a coil/sensor and/or driving it withan off-resonance signal, e.g. a low frequency or DC signal. This alsofacilitates using resonant circuit-based sensors by reducinginterference between sensors.

One or more of the above described techniques may be employed to limitinterference between nearby sensors. Which, and how many, techniques areemployed may depend in part upon the distance between the active andpassive resonant circuits when a key is up and/or the distance of travelbetween key up and key down positions. For example in a piano-stylekeyboard the travel may be in the approximate range 5 mm to 15 mmdepending upon the design. With larger distances pressing one key mayresult in seeing another nearby key move, and thus one or more of theabove techniques may be beneficially employed to ameliorate this effect.Thus in general, some implementations of the sensing system may employ amultiplexing arrangement as described herein and some additional meansto reduce interference between nearby sensors.

The sensing system may further comprise a temperature-compensationsystem to temperature-compensate the detected level of RF signal. Thetemperature-compensation system may be configured to apply anoff-resonance drive signal to at least one of the active resonantcircuits. It may then measure a level of the off-resonance drive signalfrom the at least one detector, and it may then compensate (e.g. offset)the detected level of RF signal responsive to the level of theoff-resonance drive signal. In some implementations the multiplexingsystem is configured to multiplex the drive signal such that one of thekey sensors is driven in each of a set of time slots. Then thetemperature-compensation system may be configured to apply theoff-resonance drive signal during an additional time slot, in particulara time slot not used for key interrogation.

In some implementations each key sensor may further comprises aresilient deformable element, for example below one of the resonantcircuits, for example a deformable end stop, or between the resonantcircuits, in particular to limit motion of one or both of the passiveresonant circuit and the active resonant circuit for pressure sensing,in particular by detecting motion against the resilient deformableelement.

In a related aspect there is provided a method of periodicallycompensating a response of a keyboard. Each key of the keyboard may havea sensor comprising an active resonant circuit, a passive tuned resonantcircuit and a detector. The method may comprise retrieving from storagea detected initial output signal of the sensor, O_(t0), at a first time,to, wherein at to the active resonant circuit is being driven at afrequency below a resonant frequency of the active resonant circuit. Themethod may further comprise, periodically, for at least one of thesensors, detecting a later output signal of the sensor, O_(t1), at atime after to. The method may then calculate an adjustment value, forexample a difference between the initial output signal of the sensor andthe later output signal of the sensor. The method may then furthercomprise compensating the response of the keyboard by adjusting anoperational output of the sensor using the adjustment value. Theoperational output may be an output from the sensor when the activeresonant circuit is being driven at the resonant frequency of the activeresonant circuit. The method may further comprise operating the sensoraccording to a time division multiplexed addressing scheme. The methodmay then use a “spare” time slot of the time division multiplexedaddressing scheme, in which the sensor is not operational, for thedetecting.

In another aspect there is provided a set of sensors for a keyboard, inparticular the keyboard of a keyboard instrument, in particular apiano-style keyboard. The keyboard has a plurality of keys. The set ofsensors may be part of a sensing system. Each sensor may comprise apassive resonant circuit for mounting on a moving part of a key and anactive resonant circuit for mounting in a fixed, reference position, forexample on part of the keyboard or instrument. In implementations thepassive resonant circuit has a resonant frequency and the activeresonant circuit excites the passive resonant circuit at the resonantfrequency. Each sensor may further comprise a detector, which may beshared between multiple sensors, to detect variation of a resonantsignal in the active resonant circuit with relative position of theactive and passive resonant circuits to thereby detect a position and/orvelocity of the key. The variation may, in some implementations, be avariation in amplitude of signal in the resonant signal. The set ofsensors may comprise sensors having two or more different resonantfrequencies arranged such that sensors having the same resonantfrequency are non-adjacent when mounted to sense keys of the keyboard.

Embodiments of this approach can be relatively inexpensive to constructbut are also reliable and not prone to the key bounce of mechanicalswitches, which in turn enables them to respond to key movements veryquickly and reliably. For example ideally each key would be measured ata rate of at least 250 times per second, and on an 88-note keyboard thiscorresponds to 22,000 keys/second. Some implementations of the describedsystem can operate at well over ten times this speed. Embodiments of thesystem can also provide excellent temperature stability, and arenon-contact so robust and substantially immune to contamination. Someimplementations of the sensors are further able to determine a keyposition as it moves between key pressed and key released positions, andmay provide a substantially continuous determination of key position.The reference position may be a fixed position beneath the key, forexample on a keyboard base or mount or it may be a position on a printedcircuit board (PCB) carrying the set of sensors for the keyboard.Alternatively however, in some implementations the active resonantcircuit may be mounted on or in association with a key and the resonantcircuit may be mounted on the base, PCB or similar.

Some implementations of the sensors are also able to detect when a keymoves beyond a key pressed position, and hence are useful inimplementing polyphonic aftertouch. Aftertouch allows a musician toapply force to a key after depression to add expression to a note bymodifying the note, for example to control volume, vibrato and the like.In some implementations polyphonic aftertouch allows a musician tocontrol this expression for each key separately.

The sensors can further sense key velocity, and/or sensed key velocitymay be employed to determine key position. Sensing key velocity mayallow further expression to be added.

In some implementations sensors having a first resonant frequency areinterleaved with sensors having a second, different resonant frequency,for example using alternate frequencies on alternate keys. This helps toreduce inter-sensor interference.

The set of sensors may include a controller to control selection orscanning of the sensors such that adjacent keyboard sensors are selectedat different times, again to reduce inter-sensor interference. In someimplementations the controller may damp the response of active resonantcircuits of unselected sensors, for example by connecting part of theactive resonant circuit to ground, for example via a resistor. Thecontroller may comprise a multiplexing system and/or a microprocessor.

In some implementations the controller/multiplexing system may beconfigured to time division multiplex operation of the sensors. In suchan approach each resonant frequency may define a group of sensors, andthe time division multiplexing may define a plurality of n time slots.Successive keyboard sensors, for example of each group, are allocatedsuccessive time slots. The successive sensors, for example of eachgroup, may be non-adjacent on the keyboard if sensors of the groups ofsensors are interleaved. There may be N resonant frequencies and thus Ngroups of sensors; In some implementations N=1. In some implementations,after activating a sensor of a current group of sensors in a currenttime slot the controller may in the next time slot activate the nextsensor along the keyboard which is in the same group of sensors.

Preferably the controller/multiplexing system is configured such thatadjacent sensors are not active simultaneously, althoughnext-to-adjacent sensors may be active simultaneously. The spacingbetween simultaneously active sensors may be (m×N)+1 where m is in therange 1 to n/2; higher separations are preferred (where a spacing of 1refers to adjacent sensors).

The closest physical spacing for simultaneously active sensors in thesame group may be a spacing of n×N sensors, later referred to as asubset of sensors, since typically a keyboard will have more than onesuch subset. Thus the controller/multiplexing system may be configuredsuch that keyboard sensors in the same group and activated in the sametime slot have (n×N)−1 sensors between them. In some implementations nmay be 8 and N may be 2.

The controller may be implemented using a processor coupled to anaddressing device such as a digital demultiplexer to address thesensors; a signal may be read from the addressed sensors by selectivelyconnecting a sensor active resonator to a read-out circuit via ananalogue multiplexer. The detector, i.e. read-out circuit, may performan envelope detect function. The read-out circuit and/or analoguemultiplexer may be enabled by an enable signal derived from a drivesignal to an active resonator, in some implementations via an adjustablephase shift. The adjustable phase shift may be used, in the context ofor separately from such a demultiplexer-multiplexer arrangement, toimplement synchronous detection of the signal from an active resonantcircuit.

The controller or another processor may be configured to process thevariation of the resonant signal in the active resonant circuit of eachsensor to determine the motion of each key of the keyboard over asuccession of time intervals as a depressed key moves between releasedand depressed positions, when a key is depressed and/or released. Themotion of each key may comprise a position and/or an approximatevelocity of the key as the key moves between released and depressedpositions.

In some approaches the position of a key may be determined from thevelocity of a key, for example by integration, rather than directly. Theprocessor may output data defining a profile of approximate positionand/or velocity over time for each key or for each moving key.

In some implementations the processor is configured to process thevariation of the resonant signal in the active resonant circuit of eachsensor to determine the approximate velocity of a key from changes inposition of the key determined at successive time intervals. A velocitydetermined in this way may be filtered dependent upon key velocity, forexample applying greater filtering/smoothing when a key is movingslowly. This helps to provide accurate data when a key is moving slowlywithout significantly compromising the response time for a fast-movingkey.

More generally a processor may process the amplitude and/or othervariation of the resonant signal to determine a key press and keyrelease event for each key, for example from a determination of keyposition and/or velocity. The processor may thus output a note signalfor each key/each active key.

In some approaches the succession of key positions or key movementprofile may be used to predict when a pressed (or released) key reachesa key-press/note-on (or key-release/note-off) position, for example byextrapolating a trajectory of the key position. The predicted positionmay be the position later referred to as K. The processor may then issuea key-press/note-on (or key-release/note-on signal in advance of theactual key-press/note-on (or key-release/note-on position being reached.This can be advantageous for compensating in processing delays in theinstrument, for example latency in the sound generation engine.

In some implementations the succession of key positions or key movementprofile may be used to provide signals to control the instrument, forexample to control the sound generation engine to add expression to thesounds, before and/or after a note-on event has been issued.

In some implementations the processor may be further configured todistinguish between at least three different key positions, a first,note-off position, a second, note-on position, and a third, aftertouchposition. The aftertouch position may be beyond the note-on position andcorrespond to additional pressure applied to the key after depression.The processor may determine a position and/or velocity of the key as itmoves to/from the aftertouch position, for example to act as a variablepressure sensor, or the processor may simply identify when theaftertouch position is reached. The aftertouch position may correspondto motion of a key beyond its usual depressed position as a result ofthe application of additional pressure to the key. Each key may beprovided with a resilient bias or deformable end-stop device such as acompression or tension spring or compressible element or block, so thaton depression part of the key interacts with the device and is inhibitedfrom further motion by the device unless additional pressure is appliedto the key, whereupon the key moves towards its aftertouch position. Anaftertouch position may be detectable for each key to provide apolyphonic aftertouch function.

In some implementations of the above described systems a new note-onsignal may be issued before a note-off is detected, to facilitatere-triggering, which is useful for piano keyboards. A pressure-controlkey movement distance (dead-zone) may be provided between a maximumkey-pressed position and the start of aftertouch detection, for exampleto allow the amount of pressure required before aftertouch begins to beconfigured.

The set of sensors may be provided on a substrate such as a printedcircuit board. The sensors may be disposed linearly along the substrate,in particular at locations which correspond to locations of keys of thekeyboard, more specifically adjacent where the passive resonant circuitsare located on the keys. Coils for the active resonant circuits may beformed by tracks on the substrate, for example defining pancake coils.Although a piano keyboard has black and white keys the sensors may beplaced in a single line when the passive resonant circuits areappropriately located. However in some implementations the active andpassive resonant circuits are alternate between two positions and arealternately displaced to either side of a longitudinal line defining ageneral arrangement of the sensors. A set of sensors may comprisesensors for a complete keyboard or for part of the length of a keyboard,for example one, two or more octaves. There is also provided a keyboardfor a keyboard instrument comprising one or more the sets of sensors aspreviously described.

In general a processor/controller of the set of sensors may be any sortof processing device/circuitry, for example comprising one or more of: amicroprocessor under program code control, or a digital signal processor(DSP), or hardware such as an FPGA (field programmable gate array) orASIC (application specific integrated circuit). In some implementationsthe control/processing functions for a set of sensors may be provided ina single integrated circuit.

Where a programmable device is employed the processor may haveassociated working memory and non-volatile program memory storingprocessor control code to control the processor to implement some or allof the functions described above. Thus there is also provided anon-transitory data carrier, such as non-volatile memory, carrying codeand/or data to implement functions described above. The code/data maycomprise source, object or executable code in a conventional programminglanguage, interpreted or compiled, or assembly code, code/data forsetting up or controlling an ASIC or FPGA such as code for a hardwaredescription language such as Verilog (Trade Mark). As the skilled personwill appreciate such code and/or data may be distributed between aplurality of coupled components in communication with one another.

There is also provided a method of sensing the positions of a pluralityof keys, for example of a keyboard instrument. The method may compriseproviding each key with a sensor comprising a passive resonant circuitfor mounting, for example, on a moving part of a key and an activeresonant circuit for mounting, for example, in a fixed, referenceposition, for example part of the keyboard or instrument. In someimplementations the passive resonant circuit has a resonant frequency,the active resonant circuit exciting the passive resonant circuit at theresonant frequency. Each sensor may further have a detector, which maybe shared, to detect variation of a resonant signal in the activeresonant circuit with relative position of the active and passiveresonant circuits to detect a position and/or velocity of the key. Themethod may further comprise arranging the sensors to operate at two ormore different resonant frequencies arranged such that keyboard sensorshaving the same resonant frequency are non-adjacent. Additionally oralternatively the method may further comprise reducing interferencebetween sensors by configuring one or more coils of at least the activeresonant circuits, and optionally also of the passive resonant circuits,to have windings in opposite senses.

The method may further comprise providing polyphonic aftertouch bydistinguishing between at least three different key positions, a first,note-off position, a second, note-on position, and a third, aftertouchposition, wherein the aftertouch position is beyond the note-on positionand corresponds to additional pressure applied to the key afterdepression and movement of a key beyond an end-stop position.

There is further provided a keyboard, in particular a musical keyboardproviding an output signal derived from measurements of the position andof the velocity and of the pressure applied to a plurality of moveablekeys on the keyboard. The measurements may be derived from positionsensors on the moveable keys. Each position sensor may comprise anactive tuned resonant circuit; drive electronics coupled to the activetuned resonant circuit to drive the active tuned resonant circuit,optionally shared between sensors; and an electrically reactive elementassociated with the moveable key. The electrically reactive element mayprovide a variable modification to a response of the active tunedresonant circuit dependent on a relative position of the electricallyreactive element with respect to the active tuned resonant circuit. Thekeyboard may further comprise read-out electronics coupled to the activetuned resonant circuit, to provide a variable output signal responsiveto the relative position of the electrically reactive element withrespect to the active tuned resonant circuit. The variable output signalof the read-out electronics may provide the position sensor output.

Preferably, but not essentially, the electrically reactive elementcomprises a passive tuned resonant circuit tuned to a frequency at whichthe active tuned resonant circuit is driven, thus the position sensor isoperated at a single resonant frequency. Advantages to this approachinclude: Firstly, a larger effective sensing distance can be achievedfor a given size of position sensor. Secondly, a larger variation in theoutput signal of the position sensor for a given variation in sensedposition can be obtained, often removing the requirement of an outputamplifier for the position sensor and thus reducing complexity and cost.Thirdly, operation of a plurality of proximally located position sensorsis facilitated because the inventors have found that a passive tunedresonant circuit of a first position sensor tuned to the resonantfrequency of the first position sensor does not substantially affect theoutput of a second position sensor if the second position sensor istuned to a significantly different resonant frequency to that of thefirst position sensor.

In broad terms an example range of resonant frequencies is 1-10 MHz,balancing speed against the deleterious effect of parasitics. Forexample a first resonant frequency may be in the range 3-4 MHz and asecond resonant frequency may be in the range 4-5 MHz.

A particularly advantageous means of forming coils used by the activetuned resonant circuit and passive tuned resonant circuit has been foundto be a flat or planar coil defined by tracks on a printed circuitboard. This helps achieve a well-defined repeatable geometry andfacilitates other electrically active components being proximallylocated on the printed circuit board.

To minimise electro-magnetic emissions radiated from the position sensorand to minimise susceptibility to electro-magnetic interference signalsof said position sensor, the coils of the active tuned resonant circuitmay be formed from a plurality of electrically connected primary“smaller” coils wherein the winding direction of said primary smallercoils is chosen such that the sum of the electro-magnetic far fieldradiated from said primary smaller coils is substantially zero. In thiscase the inductance coils used by the passive tuned resonant circuitmay: be inductively coupled to only a subset of said primary smallercoils; or be comprised of a plurality of electrically connectedsecondary smaller coils wherein the winding direction and number of saidsecondary smaller coils may be chosen to maximise the variation in theoutput signal of said position sensor.

Although the above described systems and methods are particularlyadvantageous for use with keyboards their applications are notrestricted to keyboards.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,with reference to the accompanying drawings, in which:

FIG. 1 shows an active tuned resonant circuit for use with exampleimplementations of the system.

FIG. 2 shows a passive tuned resonant circuit for use with exampleimplementations of the system.

FIG. 3A shows an example printed circuit design, on an enlarged scale,for an active tuned resonant circuit for use with exampleimplementations of the system.

FIG. 3B shows an example printed circuit design, on an enlarged scale,for a passive tuned resonant circuit for use with exampleimplementations of the system.

FIG. 4 shows a cross section view of a key of a musical keyboard whereinthe position of a key is determined using an example implementation ofthe system.

FIG. 5 shows an example of a simple read-out electronic circuitcomprising a synchronous demodulator which may be used by exampleimplementations of the system.

FIG. 6 shows the relative positioning of a series of active tunedresonant circuits and a series of corresponding passive tuned resonantcircuits positioned to minimise interference between adjacent tunedresonant circuits according to example implementations of the system.

FIG. 7 shows a timing diagram of a time division multiplex circuit usedto multiplex a plurality of active tuned resonant circuits to determinethe position of a plurality of keys on a musical keyboard according toan example implementation of the system.

FIG. 8 shows a time division multiplex circuit diagram of a preferredembodiment used to multiplex a plurality of active tuned resonantcircuits to determine the position of a plurality of keys on a musicalkeyboard according to example implementations of the system.

FIG. 9 shows the sensor output versus key displacement of a key on amusical keyboard according to example implementations of the system.

FIG. 10 shows an example of the measured position and measured velocityof a key on a musical keyboard as it is depressed according to exampleimplementations of the system.

FIG. 11 shows an example calibration procedure used to calibrate thedetected position of a key on a musical keyboard according to exampleimplementations of the system.

FIG. 12 shows an example algorithm used to detect note-on events,note-off events, expression events, and pressure events for a key on amusical keyboard according to example implementations of the system.

FIGS. 13A and 13B shows examples of sensor resonant circuits with coilshaving windings in opposite senses, according to example implementationsof the system.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment comprises a musical keyboard with a plurality ofmoveable keys wherein each moveable key FIG. 4 comprises: a moveable topmember 15 that is rotated about a pivot point 17 and which resistsmovement by means of a spring 16 or other mechanical linkage; a fixedbottom member 14; a deformable end-stop 18 which limits movement of saidtop member; and a position sensor comprising an active tuned resonantcircuit 10 inductively coupled to an electrically reactive element 11,henceforth referred to as the target, providing a signal which varies asthe mutual separation of said active tuned resonant circuit and saidtarget is varied, drive electronics connected to said active tunedresonant circuit and read-out electronics connected to said active tunedresonant circuit.

The active tuned resonant circuit FIG. 1 comprises an input resistiveelement 4, a coil 1, two capacitive elements 2 and 3, an outputresistive element 5, a means of connecting 6 drive electronics to saidinput resistive element and a means of connecting 7 read-out electronicsto said output resistive element. Said input resistive element may beomitted, but it is preferred because: it limits the current supplied tosaid active tuned resonant circuit from said drive electronics whichreduces the operating current and thus reduces both power consumptionand electro-magnetic emissions from said active tuned resonant circuit;and it increases the sensitivity of proximity detection when saidread-out electronics are connected to said active tuned resonantcircuit. Said output resistive element may be omitted, but it is alsopreferred because said input and output resistive elements reduce theeffect of connecting wires on the impedance of said active tunedresonant circuit thus allowing all the position sensors to beessentially the same regardless of the length of connections to thedrive electronics and to the read-out electronics.

Referring to FIG. 2, the reactive element preferably comprises a passivetuned resonant circuit which comprises a coil 8 and a capacitive element9 wherein said coil and said capacitive element are connected to form aclosed resonant LC circuit. It is not necessary for the size nor for thevalue of inductance of the coils 1 and 8 to be substantially similar.The value of the capacitance of said capacitive element 9 is preferablychosen to tune the frequency of resonance of said passive tuned resonantcircuit to match the frequency of resonance of the active tuned resonantcircuit FIG. 1. When said passive and active circuits are thus tuned, itis possible to operate a plurality of position sensors where proximallylocated said position sensors are tuned to substantially differentfrequencies of resonance thereby minimising the interaction between saidproximally located position sensors. Furthermore when said passive andactive circuits are thus tuned the signal amplitude at 7 in FIG. 1decreases as the distance between said passive and active circuitsdecreases because more energy is coupled to and dissipated by saidpassive tuned resonant circuit. Such variation in said signal amplitudeis preferred because measuring variations in signal amplitude is fasterthan measuring variations in frequency of resonance as would beimplemented in the case where said active tuned resonant circuit wasdetuned by proximity to said reactive element.

In the case where the moveable top member 15 of a key comprises anelectrically conductive material, and air gap or spacer 13 comprised ofa non-conductive material is interposed between said electricallyreactive element 11 and said top member. Similarly, in cases there thefixed bottom member 14 comprises an electrically conductive material,and air gap or spacer 12 comprised of a non-conductive material isinterposed between the active tuned resonant circuit 10 and said fixedbottom member.

The drive electronics comprise a means of generating an oscillatingvoltage drive waveform at a frequency equal to or close to the frequencyof resonance of the active tuned resonant circuit. Typically, but by wayof non-limiting example, this waveform is a square waveform generated bythe output of a microcontroller timer or a digital or analogue timingcircuit.

The read-out electronics comprise a means of generating a voltageproportional to the amplitude of the signal at the read-out point 7.Typically, but by way of non-limiting example, this comprises asynchronous demodulator circuit FIG. 5 wherein the signal from saidread-out point is connected to 20 and demodulated by an analogue switch22 controlled by the oscillating voltage drive waveform connected to 19whose phase is optionally adjusted by a phase shifting element 21 and alow-frequency (or dc) voltage is presented at 25 by a low-pass filtercomprising a resistive element 23 and a capacitive element 24.Alternative read-out electronic circuits may comprise phase-sensitiverectifiers, phase-insensitive rectifiers, non-synchronous demodulatorsand peak detectors as understood by those trained in the art.

The coils 1 and 8 used in the active tuned resonant circuit and thepassive tuned resonant circuit respectively can be of any type. Howeverusing planar spiral coils formed by tracks on a printed circuit boardhas three main advantages: they are inexpensive, they can be made withhighly reproducible values of inductance and the printed circuit boardcan also be used to mount the other components, namely the capacitiveelements 2, 3 and 9, and the resistive elements 4 and 5. It is thereforepossible to design a plurality of coils whose inductance values areclosely matched.

Referring to FIG. 3A, a typical active tuned resonant circuit may beformed on a printed circuit board comprising a single electricallyconductive layer or a plurality of electrically conductive layerswhereon: the coil 1 is formed of a continuous spiral track wherebyelectrical continuity of said track is maintained by electricalconnection through connecting vias 53 to a connecting wire or to anotherspiral track on another conductive layer or to a plurality of spiraltracks on a plurality of conductive layers of said printed circuitboard; capacitive elements 2 and 3 and resistive elements 4 and 5 areproximally located; and connection points 6 and 7 are provided for driveelectronics and read-out electronics, respectively.

Similarly, referring to FIG. 3B, a typical passive tuned resonantcircuit may be formed on a printed circuit board comprising a singleelectrically conductive layer or a plurality of electrically conductivelayers whereon: the coil 8 is formed of a continuous spiral trackwhereby electrical continuity of said track is maintained by electricalconnection through connecting vias 54 to a connecting wire or to anotherspiral track on another conductive layer or to a plurality of spiraltracks on a plurality of conductive layers of said printed circuitboard; and the capacitive element 9 is proximally located.

The inventors have found that the electro-magnetic emissions from anactive tuned resonant circuit, and the susceptibility toelectro-magnetic interference signals of said active tuned resonantcircuit can be substantially reduced when the inductive coil of saidactive tuned resonant circuit is formed from a plurality of electricallyconnected primary smaller coils wherein the winding direction of saidprimary smaller coils is chosen such that the sum of theelectro-magnetic far field radiated from said primary smaller coils issubstantially zero. A particularly suitable, but by way of non-limitingexample, of said inductive coil 1 is shown in FIG. 13A, wherein twoprimary smaller coils are wired in series with opposing windingdirections 58 to form a figure-of-eight coil. In such an arrangement theelectro-magnetic far field radiated from the first half of saidfigure-of-eight coil 56 is equal in magnitude but with opposite polarityto the electro-magnetic far field radiated from the second half of saidfigure-of-eight coil 57, thus said electro-magnetic far field radiatedfrom said figure-of-eight coil is substantially zero.

In such an arrangement, a passive tuned resonant circuit as shown inFIG. 3B may be ineffective unless the inductive coil of said passivetuned resonant circuit is primarily inductively coupled to only one half56 or 57 of the figure-of-eight coil of the active tuned resonantcircuit. To maximise the output signal of the position sensor, it ispreferable for said inductive coil of said passive tuned resonantcircuit to be similarly formed of a figure-of-eight inductive coil, asshown in FIG. 13B, comprising two secondary smaller coils wired inseries with opposing winding directions 58 wherein each said secondarysmaller coil is primarily inductively coupled to a different primarysmaller coil of said figure-of-eight coil of said active tuned resonantcircuit.

The inventors have found that although a first passive tuned resonantcircuit tuned to a first frequency of resonance of a first active tunedresonant circuit does not substantially affect the output of an adjacentsecond active tuned resonant circuit tuned to a substantially differentsecond frequency of resonance, when a corresponding second passive tunedresonant circuit tuned to said second frequency of resonance isproximally located, movement of said first passive tuned resonantcircuit may affect the output of said second active tuned resonantcircuit due to mutual coupling between said first and second passivetuned resonant circuits. Such undesirable interaction can be minimisedby offsetting the positions of physically adjacent passive tunedresonant circuits, as shown in FIG. 6 wherein: active tuned resonantcircuits 26 and passive tuned resonant circuits 28 are tuned to a firstfrequency of resonance and active tuned resonant circuits 27 and passivetuned resonant circuits 29 are tuned to a second frequency of resonance.

In a further preferred embodiment the position sensors on the moveablekeys of the musical keyboard are controlled by a time-divisionmultiplexing scheme whereby a subset of position sensors are enabled atany given time. For a typical musical keyboard with a large number ofkeys such as 16 or more, such a scheme has the advantage of reducingcost, complexity, power consumption and electro-magnetic emissions.

In the case where a first position sensor operating at first frequencyof resonance and a second position sensor operating at a substantiallydifferent second frequency of resonance are proximally located saidposition sensors can interact in such a way that the output of saidfirst position sensor and the output of said second position sensorcontains interference components which vary with a frequency ofvariation equal to the frequency difference of said first frequency ofresonance and said second frequency of resonance. Synchronousdemodulation of the output of said position sensors substantiallyremoves said interference components when the cut-off frequency of thereconstruction low-pass filter is substantially lower than saidfrequency difference. However, the time response of said low-pass filtercan limit the speed of response of said position sensors which isundesirable. Therefore, a mechanism to minimise this interference isdesired. Using a time-division multiplexing scheme where physicallyadjacent sensors are not driven at the same time avoids this problem.

In practice it has been found that synchronous demodulation is notnecessary for good performance.

Referring to FIG. 7, one illustrative example of such a mechanism isshown as a timing diagram for a subset of position sensors whereinposition sensors 30 operating at a first frequency of resonance F1 areadjacent to position sensors 31 operating at a second frequency ofresonance F2. In each time slot only one position sensor operating at afirst frequency of resonance is enabled and only one position sensoroperating at a second frequency of resonance is enabled. Furthermore,physically adjacent position sensors are never enabled at the same time,minimising said interference components. A plurality of said subsets ofposition sensors can be operated simultaneously.

In broad terms, in the multiplexing of FIG. 7 the keys illustratedshaded black and the keys illustrated shaded white each form a group ofkeys. The sensors in one group of keys may have a different resonantfrequency to the sensors in another group of keys. With a group, say ofthe black keys, there are 8 time slots and every 8th key is activated(driven) simultaneously. The skilled person will understand that thisapproach could be adapted for k time slots, driving every kth keysimultaneously (that is simultaneously driven keys have k−1 inactivekeys between them). Keys in simultaneously active groups, e.g. black andwhite keys, may be (physically) separated as far as possible.

Some implementations of the system do not employ different groups ofkeys with different resonant frequencies. Instead all the sensors mayhave substantially the same resonant frequency. Use of such an approachis facilitated by the coil design described later with reference to FIG.13. Thus there may be k time slots and every kth key may be active(driven) simultaneously—that is simultaneously driven keys may have k−1inactive keys between them.

An example time-division multiplexed scheme is shown for a subset ofposition sensors operating at a single frequency of resonance in FIG. 8.In the system of FIG. 8 a processor 35 generates a drive waveform 36whose frequency matches the frequency of resonance of said positionsensors' active tuned resonant circuits; said processor generatesselector signals 37 to select which position sensor is to be enabled;said position sensors' outputs 7 are coupled to an analogue multiplexer34; said analogue multiplexer's output is coupled to ananalogue-to-digital converter within said processor via a low-passfilter comprising a capacitive element 24 and resistive element withinsaid analogue multiplexer; and an output 55 from said processor is usedto send information regarding the position and velocity of said positionsensors. A further advantage to using said analogue multiplexer tocouple said position sensors' outputs to said analogue-to-digitalconverter is that said analogue multiplexer can perform the function ofthe analogue switch 22 used for synchronous demodulation whereby theoutput of said analogue multiplexer can be synchronously enabled anddisabled via an enable input 39 coupled to said drive waveform 36. Inthe case where a plurality of position sensors are operated atsubstantially different frequencies of resonance said time-divisionmultiplexed scheme can be replicated as necessary. A suitable processoris an ARM Cortex-MO.

FIG. 8 shows just one demultiplexer/multiplexer but if there aremultiple resonant frequencies one demultiplexer/multiplexer may beemployed for each of the resonant frequencies used. For example a seconddemultiplexer/multiplexer may be used where alternate resonantfrequencies are mapped to alternate keys of the keyboard.

Decreased sensitivity to detuning of the position sensor's active tunedresonant circuit or passive tuned resonant circuit, for example, causedby variations of component tolerance, may be facilitated by coupling theoutput of the (optional) synchronous demodulator circuit to a peakdetection circuit comprising a diode 40 a capacitive element 24 andoptionally a resistive element 41 or a switching element 42 (to resetthe charge on capacitive element 24). In the case where a switchingelement is used said switching element may reset the detected peak levelsynchronously with the selector signals used to control themultiplexers.

The signal from the detector (read-out circuitry) may be input to ananalogue-to-digital converter 38, for example integrated into ananalogue input of processor 35.

In the case where a disabled position sensor's active tuned resonantcircuit is not being driven, said active tuned resonant circuit acts asa tuned antenna. This has the negative effect whereby moving the targetcorresponding to said disabled position sensor can effect a measurablevariation in the output of a similarly-tuned position sensor even ifsaid similarly-tuned position sensor is not physically adjacent to saiddisabled position sensor and the motion of said target is constrained tobe within its normal limits above said disabled position sensor,according to FIG. 4 and FIG. 6. Said negative effect can be reduced bychanging the frequency of resonance of said disabled position sensor'sactive tuned resonant circuit for the duration of the disablement, forexample by changing the capacitance, resistance or inductance of saidactive tuned resonant circuit by electronic switching. Most simply thiscan be done by driving said disabled sensor with a direct-current, orlow-frequency signal, to prevent resonance. Referring to FIG. 8, aparticularly advantageous way to achieve this in a time-divisionmultiplexed scheme is to use a digital demultiplexer 33 to drive theinputs 6 of the active tuned resonant circuits whereby enabled positionsensors' active tuned resonant circuits are driven by a waveform 36 atthe frequency of resonance of said active tuned resonant circuits anddisabled position sensors' active tuned resonant circuits are driven bya direct-current signal corresponding to logic-high or logic-low of saiddigital demultiplexer.

It is important for the performance of a musical keyboard to be stableover a range of operating temperatures. Although the tuned resonantcircuits used by a position sensor as described herein have excellenttemperature stability, particularly when the tuned resonant circuits areformed on a printed circuit board and the capacitive elements of thetuned resonant circuits comprise temperature-stable dielectrics (Class 1dielectrics), other electronic elements in the circuit can haveproperties that change with temperature which may cause a variation inthe output signal of the position sensor with variations in operatingtemperature. Such electronic elements include but are not limited to:diode 40, digital demultiplexer 33, analogue multiplexer 34, resistiveelements 4, 5 and 41, tracks on printed circuit boards, and voltageregulators. Therefore a temperature compensation scheme can be useful tominimise variations in the output signals of a plurality of positionsensors on a musical keyboard caused by variations in operatingtemperature.

A particularly suitable, but by way of non-limiting example, temperaturecompensation scheme comprises: performing measurements of the outputsignal of a position sensor while driving said position sensor's activetuned resonant circuit with a direct-current, or low-frequency signalsuch that said position sensor's passive tuned resonant circuit has noeffect on the output signal of said position sensor; the first of saidmeasurements is performed during a calibration procedure; the subsequentsaid measurements may be performed periodically, for example withinadditional time slots of a time-division multiplexed scheme; calculatingtemperature-dependent offsets in said output signal by subtractingsubsequent said measurements from said first measurement; and addingsaid offsets to the measurement of said output signal when said activetuned resonant circuit is being driven at a frequency equal to or closeto the frequency of resonance of said active tuned resonant circuit tomeasure position. Such a temperature compensation scheme may utilise onetemperature-dependent offset for: each position sensor in a musicalkeyboard; each group of position sensors in a musical keyboard; or forall position sensors in a musical keyboard.

A musical keyboard with moveable keys utilising a multiplexing scheme ashereinabove described allows fast and accurate measurement of theposition of said keys. For example it is possible to multiplex theexample shown in FIG. 8 wherein the frequency of update of selectorsignals 37 is at least 32,000 Hz thus allowing the position of eachmoveable key in a subset of 8 moveable keys to be determined at afrequency of 4,000 Hz. This example can be replicated and run inparallel for other subsets of moveable keys, thus allowing a full-sizepiano keyboard with 88 keys to have the position of the keys determinedat a rate of at least 352,000 keys/second. The inventors hereof havefound that positions of said keys should ideally be determined at least250 times per second, corresponding to a rate of at least 22,000keys/second for 88 keys, to allow suitably accurate timing of note-onevents and note-off events and to determine the key velocity associatedwith said events. Clearly implementations of the described system easilyexceed these targets.

Referring to FIG. 9, when a moveable key on a musical keyboard accordingto example implementations of the system is depressed there are threeprimary positions of said key: the resting position Kmax 43 when saidkey is at rest; the point Kzero 44 when the moveable top member 13 makesa first contact with the deformable end-stop 18; and the point ofmaximum depression Kmin 45, corresponding to the point of maximumpressure being applied to said key by a typical musician wherein thedeformable end-stop 18 may be considered to be maximally deformed. For aplurality of such moveable keys, due to mechanical variation and due toelectronic component tolerance, it is unlikely that the output signal ofthe position sensor of a first key at any one of said primary positionsof said first key will be identical to the output signal of the positionsensor of a second key at the same primary position of said second key.Therefore a calibration procedure is necessary to ensure that theposition of any moveable keys is known relative to the respectiveprimary positions of said moveable keys. Such a calibration procedure isshown in FIG. 11.

In the case where the position of a moveable key is between primarypositions Kmax and Kzero, the calibrated position K of said key as apercentage of depression between Kmax and Kzero can thus be calculatedfrom the measured position Ko of said key using the following equation:K=100%×(Ko−Kzero)/(Kmax−Kzero).

In the case where the position of a moveable key is between primarypositions Kzero and Kmin, the calibrated position Kpress of said key asa percentage of depression between Kzero and Kmin, 50 in FIG. 9, canthus be calculated from the measured position of said key Ko using thefollowing equation: Kpress=100%×(Ko−Kmin)/(Kzero−Kmin) In such a caseKpress may be considered to be the amount of pressure being applied tosaid key, corresponding to the range of depression 50 of said key.

In some embodiments the calculation of Kpress may include an offset,Kpoff, whereby Kpress is zero until the position of the key Ko liesbetween (Kzero−Kpoff) and Kmin; thenceKpress=100%×(Ko−Kmin)/(Kzero−Kpoff−Kmin) Said offset creates a dead-zonewherein variation in position of said key results in no variation ofcalibrated position K of said key and in no variation of Kpress. Thisfacilitates implementation of an aftertouch threshold.

On a typical musical keyboard it is desirable for each moveable key onsaid keyboard to issue a note-on event when the depression of said keyis beyond a secondary position Kon and to issue a note-off event whenthe depression of said key is returned to another secondary positionKoff. In some cases Kon may equal Koff, but it is preferred for Kon andKoff to be unequal. Referring to FIG. 9, preferably secondary positionKon 48 is chosen to be near the primary position Kzero 44. Similarly,the secondary position Koff 47 is chosen to be near the said secondaryposition Kon.

In some embodiments it is possible after issuing a first note-on eventto issue a second note-on event, and optionally issue a note-off eventpreceding said second note-on event, when the depression of saidmoveable key has returned to a position before secondary position Konbut has not returned to secondary position Koff and then the depressionof a moveable key varies to a position beyond secondary position Kon.This facilitates re-triggering.

In some embodiments the secondary position Koff 46 of each moveable keyis chosen to be near the primary position Kmax 43. Such an arrangementallows the position of said key to be used to issue expression eventsprior to issuing a note-off event wherein the measured position Ko ofsaid key between Koff and Kzero can be used to calculate a calibratedexpression value Kexp=100%×(Ko−Kzero)/(Koff−Kzero), corresponding to therange of depression 49 of said key.

By way of non-limiting example, one particular algorithm shown in FIG.12 may be used for each moveable key on a musical keyboard according toan example implementation of the system wherein the measured position Koof said moveable key, when calibrated using primary positions Kmax,Kzero and Kmin and thence using secondary positions Kon and Koff, may beused to issue: note-on events, note-off events, expression events andpressure events for each moveable key on said musical keyboard.

A particular advantage of deriving the secondary positions Kon and Koffof a moveable key on a musical keyboard from the primary positions Kmaxand Kzero of said moveable key is that said secondary positions can bemodified easily by simple numerical calculations, allowing the responseof said musical keyboard to be changed. Moreover such modification canbe different for each individual key on a musical keyboard with aplurality of moveable keys, allowing a large range of responses to beachieved on said musical keyboard without requiring any mechanicalchanges to the musical keyboard.

To provide further expressive control of a musical sound productionsystem it is common for a musical keyboard to send velocity informationrelating to note-on events and also possibly related to note-off events.Such velocity information can be determined by measuring the separationin time between two known points of key depression, or converselymeasuring the change in said key depression at two known points in time.

In embodiments the velocity (speed and direction) of a moveable key isdetermined from a plurality of positions of said key at a plurality ofcorresponding times using averaging, filtering, or similar methods. Anexample is described in detail below. Such a method of calculating saidvelocity has several advantages over other methods: it does not assume alinear velocity profile as is used for a two-point measurement methodbut allows changes in velocity throughout the range of depression ofsaid key to be detected thus measured values of velocity are morerepresentative of the true velocity of said key thus making the responseof said key more consistent; higher resolution and precision of velocitycan be determined because a larger number of statistically significantdata points are used; and it allows predictions of the future positionof said key to be calculated allowing, for example, the future time atwhich said key's position equals secondary positions Kon and Koff to beestimated, thus permitting note-on or note-off events to be issued inadvance of the corresponding physical event thus compensating forlatency in a musical sound production system.

One example filtering procedure is as follows:

deltaV=deltaPos (i.e. the change in position between fixed time steps)

alpha=k*abs(deltaV)

The filtering coefficient, alpha, depends on magnitude of deltaV; alphais limited to sensible values to avoid overflow/underflow.

velocity=alpha*deltaV+(1−alpha)*last_velocity

Such a method, which may be implemented in the digital domain, canprovide improved resolution because of the filtering, which isespecially important for a very slowly moving key, without significantlycompromising the time response for a fast-moving key. Modifying thefiltering and/or a maximum permitted velocity value can modify the feelof an instrument, for example to give it a harder or softer response.

To illustrate such benefits of such a method, FIG. 10 shows thecalibrated position 51 of a moveable key and the correspondingcalibrated velocity 52 of said key wherein the depression of said keyreaches primary point Kzero 44 within 7 ms of the start of depression ofsaid key. The plot of FIG. 10 approximates a velocity calculateddirectly from differentiated position but when the position moves slowlythe velocity filtering is heavier so the velocity lags a little. Such amethod can yield substantially more information regarding the velocityof a moveable key on a musical keyboard than other methods.

Movement detection systems for musical keyboards have been described aswell as sensing systems and methods for keyboard instruments. Howeverthe techniques described are not limited to musical keyboards and mayalso be used, for example for computer keyboards.

For example in some implementations the above described techniques maybe employed in a laptop keyboard. In this case one or both of thepassive and active resonant circuits may be mounted on a flexible PCB.For example the passive resonant circuits may be mounted beneath thekeys, on a flexible PCB and the active resonant circuits may be mountedon an underlying rigid PCB. The ability to sense position may be used tosense pressure applied to a key, for example if some resilient materialis provided between the active and passive resonant circuits. In someimplementations, for example a laptop, computer, or other keyboard,where the keys are arranged in a 2D pattern on a flat or curved surface,the multiplexing may be arranged, for example in a generallycorresponding manner to that described above, so that no key is drivenat the same time as an adjacent key in two dimensions. For example in arectangular 2D grid alternate keys in each of two dimensions in asurface defined by the keyboard may be active in alternate time slots(i.e. two sets of non-adjacent keys may be identified); this may beextended to key layouts defined by hexagonal and other grids where setsof non-adjacent keys may similarly be identified. Keys which areadjacent to one another in a surface defined by the keyboard may beinactive and/or damped when a target key is read. However, as previouslydescribed, the multiplexing may be arranged to read multiple keys of thekeyboard simultaneously. The described techniques can be advantageousfor computer and other keyboards because they can be fabricatedinexpensively and because response times can be very quick, for example<1 ms.

In another implementation, the above described techniques may beemployed to sense pressure, a sensor further comprising a deformableelement, for example a block or layer of rubber, below and/or between ofone or both of the passive resonant circuit and the active resonantcircuit. Such an arrangement may be employed, for example, as a sensorfor an electronic drum pad.

Further aspects of the invention are defined in the following clausesC1-C25:

C1. A sensing system for a keyboard, the sensing system comprising:

-   -   a plurality of key sensors, wherein each key sensor comprises:

a passive resonant circuit, and an active resonant circuit, the passiveresonant circuit having a resonant frequency, the active resonantcircuit being configured to excite the passive resonant circuit at theresonant frequency;

the sensing system further comprising:

at least one sensor driver to drive the active resonant circuits with anRF drive signal at the resonant frequency;

a multiplexing system to multiplex the drive signal such thatsimultaneously driven key sensors are separated by at least (k−1) keys,where (k−1) is an integer equal to or greater than 1; and

at least one detector to detect a level of RF signal from a driven keysensor for sensing a position and/or velocity of a key associated withthe key sensor.

C2. A sensing system as in C1 configured to damp the active resonantcircuits of key sensors which are not driven.

C3. A sensing system as in C1 or C2 wherein at least the active resonantcircuit comprises one or more coils with windings in opposite senses, inparticular wherein the windings in opposite senses are configured togenerate magnetic fields in opposite senses to cancel one another.

C4. A sensing system as in C1, C2 or C3 wherein the active resonantcircuit comprises a pair of laterally adjacent pancake coils.

C5. A sensing system as in any one of C1 to C4 further comprising atemperature-compensation system to temperature-compensate the detectedlevel of RF signal, wherein the temperature-compensation system isconfigured to apply an off-resonance drive signal to at least one of theactive resonant circuits, to measure a level of the off-resonance drivesignal from the at least one detector, and to compensate the detectedlevel of RF signal responsive to the level of the off-resonance drivesignal.

C6. A sensing system as in C5 wherein the multiplexing system isconfigured to multiplex the drive signal such that one of the keysensors is driven in each of a set of time slots, and wherein thetemperature-compensation system is configured to apply the off-resonancedrive signal during an additional time slot to the set of time slots.

C7. A sensing system as in any one of C1 to C6 wherein each key sensorfurther comprises a deformable element to limit motion of one or both ofthe passive resonant circuit and the active resonant circuit forpressure sensing.

C8. A sensing system comprising a set of sensors for the keyboard of akeyboard instrument,

wherein the keyboard has a plurality of keys;

wherein each sensor comprises a passive resonant circuit for mounting ona moving part of a key and an active resonant circuit for mounting in areference position, the passive resonant circuit having a resonantfrequency, the active resonant circuit exciting the passive resonantcircuit at the resonant frequency, each sensor further having a detectorto detect variation of a resonant signal in the active resonant circuitwith relative position of the active and passive resonant circuits todetect a position and/or velocity of the key; and

-   -   wherein the set of sensors comprises sensors having two or more        different resonant frequencies arranged such that sensors having        the same resonant frequency are non-adjacent.

C9. A sensing system as in C8 wherein sensors having a first resonantfrequency are interleaved with sensors having a second, differentresonant frequency.

C10. A sensing system as in C8 or C9 further comprising a multiplexingsystem and/or controller to control selection of sensors of the set ofsensors such that adjacent keyboard sensors are selected at differenttimes.

C11. A sensing system as in any one of C1-7 and C10 wherein themultiplexing system/controller is further configured to damp the activeresonant circuits of unselected sensors.

C12. A sensing system as in C10 or C11 wherein the multiplexingsystem/controller is configured to time division multiplex operation ofthe sensors, wherein each resonant frequency defines a group of sensorshaving the resonant frequency, wherein the time division multiplexingdefines a plurality of n time slots, and wherein successive keyboardsensors of each group are allocated successive time slots.

C13. A sensing system as in C12 wherein there are N resonant frequenciesand N groups of sensors, wherein sensors of the groups of sensors areinterleaved on the keyboard.

C14. A sensing system as in C13 wherein the multiplexingsystem/controller is configured such that keyboard sensors in the samegroup and activated in the same time slot have (n×N)−1 sensors betweenthem.

C15. A sensing system as in any preceding clause further comprising aprocessor configured to process the variation of the resonant signal inthe active resonant circuit of each sensor to determine the motion ofeach key of the keyboard over a succession of time intervals as adepressed key moves between released and depressed positions, inparticular wherein the motion of each key comprises a position and avelocity of the key as the key moves between released and depressedpositions.

C16. A sensing system as in C15 wherein the processor is configured toprocess the variation of the resonant signal in the active resonantcircuit of each sensor to determine the velocity of a key, as the keymoves between depressed and released positions, from changes in positionof the key determined at successive time intervals filtered dependentupon key velocity.

C17. A sensing system as in any preceding clause further comprising aprocessor coupled to process the level/variation of the RF/resonantsignal to determine a key press and key release event for each key.

C18. A sensing system as in any one of C15-C17 wherein the processor isfurther configured to distinguish between at least three different keypositions, a first, note-off position, a second, note-on position, and athird, aftertouch position, wherein the aftertouch position is beyondthe note-on position and corresponds to additional pressure applied tothe key after depression.

C19. A sensing system as in any preceding clause further comprising asubstrate supporting the active resonant circuits for the sensors in asequence corresponding to a sequence of keys of the keyboard.

C20. A keyboard, in particular for a keyboard instrument, comprising thesensing system of any preceding claim.

C21. A polyphonic aftertouch keyboard comprising the sensing system orkeyboard of C19 or 20, each key having a deformable end-stop, such thatthe after-touch position corresponds to movement of a key beyond anend-stop position defined by the deformable end-stop, whereinidentification of the aftertouch position for the key enables polyphonicaftertouch.

C22. A method of sensing the positions of a plurality of keys, inparticular of a keyboard instrument, the method comprising:

providing each key with a sensor comprising a passive resonant circuitfor mounting on a moving part of a key and an active resonant circuitfor mounting in a reference position, the passive resonant circuithaving a resonant frequency, the active resonant circuit exciting thepassive resonant circuit at the resonant frequency, each sensor furtherhaving a detector to detect variation of a resonant signal in the activeresonant circuit with relative position of the active and passiveresonant circuits to detect a position and/or velocity of the key; and

-   -   arranging the sensors to operate at two or more different        resonant frequencies arranged such that keyboard sensors having        the same resonant frequency are non-adjacent; and/or    -   reducing interference between sensors by configuring one or more        coils of at least the active resonant circuits to have windings        in opposite senses.

C23. A method as in C22 further comprising providing polyphonicaftertouch by distinguishing between at least three different keypositions, a first, note-off position, a second, note-on position, and athird, aftertouch position, wherein the aftertouch position is beyondthe note-on position and corresponds to additional pressure applied tothe key after depression and movement of a key beyond an end-stopposition.

C24. A method of periodically compensating a response of a keyboard, thekeyboard comprising keys with each with a sensor comprising an activeresonant circuit, a passive tuned resonant circuit and a detector, themethod comprising:

retrieving from storage a detected initial output signal of the sensor,O_(t0), at a first time, t_(o), wherein at to said active resonantcircuit is being driven at a frequency below a resonant frequency ofsaid active resonant circuit; and periodically, for at least one of thesensors:

-   -   detecting a later output signal of the sensor, O_(t1), at a time        after to;

calculating an adjustment value, wherein the adjustment value is adifference between the initial output signal of the sensor and the lateroutput signal of the sensor; and

-   -   compensating the response of the keyboard by adjusting an        operational output of the sensor using the adjustment value,        where the operational output is an output from the sensor when        the active resonant circuit is being driven at the resonant        frequency of the active resonant circuit.

C25. The method of C24 further comprising operating the sensor accordingto a time division multiplexed addressing scheme, and using a time slotof the time division multiplexed addressing scheme in which the sensoris not operational for the detecting.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

1. A sensing system comprising a set of sensors for the keyboard of akeyboard instrument, wherein the keyboard has a plurality of keys;wherein each sensor comprises a passive resonant circuit for mounting ona moving part of a key and an active resonant circuit for mounting in areference position, the passive resonant circuit having a resonantfrequency, the active resonant circuit exciting the passive resonantcircuit at the resonant frequency, each sensor further having a detectorto detect a variation of a resonant signal in the active resonantcircuit with a relative position of the active and passive resonantcircuits to detect a position and/or velocity of the key; and whereinone or more coils of at least the active resonant circuits to havewindings in opposite senses to thereby reduce interference betweensensors of adjacent keys.
 2. A sensing system as claimed in claim 1,wherein the system further comprises a processor configured todistinguish between at least three different key positions, a first,note-off position, a second, note-on position, and a third, aftertouchposition, wherein the aftertouch position is beyond the note-on positionand corresponds to additional pressure applied to the key afterdepression and movement of a key beyond an end-stop position.
 3. Asensing system as claimed in claim 1 wherein: one or more of the activeresonant circuits comprise more than one coil; and the more than onecoils are positioned next to one another along the longitudinaldirection defined by the key, wherein the positioned coils define amajor length along the longitudinal direction and define a minor lengthperpendicular to the longitudinal direction.
 4. A sensing system asclaimed in preceding 1 claim wherein the one or more coils of the activeresonant circuit comprise two primary coils with opposing windingdirections.
 5. A sensing system as claimed in claim 1 wherein one ormore coils of the passive resonant circuit have windings in oppositesenses.
 6. A sensing system as claimed in claim 5 wherein the one ormore coils of the passive resonant circuit comprise two secondary coilswired in series with opposing winding directions.
 7. A sensing system asclaimed in claim 6 wherein the secondary coils of the passive resonantcircuit are primarily inductively coupled to different coils of theactive tuned resonant circuit respectively.
 8. A sensing system asclaimed in claim 1 wherein active resonant circuits of adjacent keys areoffset relative to an axis defined by the arrangement of the pluralityof keys.
 9. A sensing system as claimed in claim 8 wherein an activeresonant circuit of a first key of the plurality of keys and an activeresonant circuit of a second key of the plurality of keys to be oneither side of said axis are offset on either side of said axis.
 10. Amethod of sensing the position and/or velocity of a plurality of keys,in particular of a keyboard instrument, the method comprising: providingeach key with a sensor comprising a passive resonant circuit formounting on a moving part of a key and an active resonant circuit formounting in a reference position, the passive resonant circuit having aresonant frequency, the active resonant circuit exciting the passiveresonant circuit at the resonant frequency, each sensor further having adetector to detect a variation of a resonant signal in the activeresonant circuit with a relative position of the active and passiveresonant circuits to detect a position and/or velocity of the key; andreducing interference between sensors of adjacent keys by configuringone or more coils of at least the active resonant circuits to havewindings in opposite senses.
 11. A method as claimed in claim 10 furthercomprising providing polyphonic aftertouch by distinguishing between atleast three different key positions, a first, note-off position, asecond, note-on position, and a third, aftertouch position, wherein theaftertouch position is beyond the note-on position and corresponds toadditional pressure applied to the key after depression and movement ofa key beyond an end-stop position.
 12. A method as claimed in claim 10wherein: one or more of the active resonant circuits comprise more thanone coil; and configuring the more than one coil comprises positioningeach of the coils next to one another along the longitudinal directiondefined by the key, wherein the positioned coils define a major lengthalong the longitudinal direction and define a minor length perpendicularto the longitudinal direction.
 13. A method as claimed in claim 10further comprising configuring the one or more coils of the activeresonant circuit to comprise two primary coils with opposing windingdirections.
 14. A method as claimed in claim 10 further comprisingconfiguring one or more coils of the passive resonant circuit to havewindings in opposite senses.
 15. A method as claimed in claim 10 furthercomprising configuring the one or more coils of the passive resonantcircuit to comprise two secondary coils wired in series with opposingwinding directions
 16. A method as claimed in claim 15 whereinconfiguring the one or more coils of the passive resonant circuit suchthe secondary coils of the passive resonant circuit are primarilyinductively coupled to different coils of the active tuned resonantcircuit respectively.
 17. A method as claimed in claim 10 furthercomprising offsetting, relative to an axis defined by the arrangement ofthe plurality of keys, active resonant circuits of adjacent keys.
 18. Amethod as claimed in claim 17 wherein the offsetting comprisespositioning an active resonant circuit of a first key of the pluralityof keys and an active resonant circuit of a second key of the pluralityof keys to be on either side of said axis.